sys_readdir and memfs_readdir closer to targeted look and feel.

We now have a single dirbuf of size PAGE_SIZE kept on the vnode. This is
to be used for directory contents only. The reason it's kept on FS0 is
because the contents are modified by calls such as mkdir or create, and otherwise
these would have been handled by mm0 on the page cache buffers, which wouldn't work.

tasks/fs0/include/vfs.h

@@ -10,10 +10,11 @@
  * that fs0 maintains. All other file data is in mm0 page cache.
  */
 struct dirbuf {
-	struct list_head list;
-	unsigned long bufsize;
-	u8 *buffer;
+	unsigned long npages;
+	int dirty;
+	u8 *buf;
 };
+
 extern struct list_head vnode_cache;
 extern struct list_head dentry_cache;
 
@@ -54,6 +55,7 @@ static inline struct dentry *vfs_alloc_dentry(void)
 	INIT_LIST_HEAD(&d->child);
 	INIT_LIST_HEAD(&d->children);
 	INIT_LIST_HEAD(&d->vref);
+	INIT_LIST_HEAD(&d->cache_list);
 
 	return d;
 }
@@ -68,9 +70,7 @@ static inline struct vnode *vfs_alloc_vnode(void)
 	struct vnode *v = kzalloc(sizeof(struct vnode));
 
 	INIT_LIST_HEAD(&v->dentries);
-	INIT_LIST_HEAD(&v->state_list);
 	INIT_LIST_HEAD(&v->cache_list);
-	list_add(&v->cache_list, &vnode_cache);
 
 	return v;
 }